The London Underline, proposed by the San Francisco-based design agency Gensler, would turn London's disused underground spaces into a network of pedestrian and cycle paths. The system would even convert the friction created by footsteps and bike tyres into electricity. So it's sustainable, too.

O'Sullivan talks about some of the areas that might be transformed. There's a defunct branch of the Piccadilly Line that runs from Holborn to Aldwych, and an "even longer artery" that connects Green Park to Charing Cross. He says these are the two Gensler has a "serious eye on."

The two "serious" lines for development. The blue line is an underground tunnel, and the orange an overground walkway.
Gensler/YouTube

There's no timeframe for when these underground cycle paths could become a reality, but here's video explaining Gensler's vision for the future: